1. Field of the Invention
The present invention relates generally to the field of mode discrimination means in laser cavities, and in particular, mode discrimination in macroscopic cavities wherein a vast number of modes may otherwise be sustained.
2. Description of the Related Art
The present invention relates generally to the field of lasers and optical resonator design, and in particular, to the fields of disk and spherical lasers. Also, the invention relates to cavity structure designs that utilize multi-layer dielectric (MLD) thin film reflectors that provide a high degree of mode selection.
Laser cavities of the disk and sphericaI geometries have become an increasingly intensive field of research; in particular, for such lasers that are fabricated on a miniature or microscopic scale. In the latter case, the predominant means of cavity reflection is through total internal reflection (TIR), which provides an extremely high cavity Q. Such reflective means normally manifest in xe2x80x9cwhispering modes,xe2x80x9d which propagate at angles below the critical angle for TIR. These microdisk and microsphere lasers are very effective in cases involving evanescent coupling to an adjacent dielectric structure; however, they are known to contain a very large number of competing high-order modes.
In addition, the coupling of these whispering modes for useful work is difficult for applications not utilizing evanescent coupling.
In recent years, theoretical studies have been performed on the development of derivation methods for cylindrical and spherical multilayer structures, which are aimed at providing an accurate description of the reflection coefficients and modal characteristics of these cavities. These studies address circular confinement structures with cavity dimensions on the order of the wavelengths studied. However, none of these studies are found to address the issues of applying similar circular Bragg reflectors for larger cavities of the scale used for gas and larger solid state cavities. Furthermore, these previous studies also entertain only the use of conventional MLD filters, with a large real refractive index difference, nHxe2x88x92nL=xcex94n greater than 1, for the layer pairs, and with an accordingly small number of layers required for high reflection.
The use of interference structures to enable high spectral resolving power in reflecting coatings has been described by Emmett (U.S. Pat. No. 4,925,259), wherein a very large number of alternating dielectric layers possessing a very small difference in refractive indices is used for application in high power flashlamps. The described coatings are utilized primarily for providing a high damage threshold to the high irradiance experienced in the flashlamp enclosure, as well as for obtaining a well-resolved pump wavelength for use in the described flashlamp.
The control of transverse modes in semiconductor lasers, primarily VCSEL""s, has been reported by several research groups in the last decade. These latter reports utilize a circular Bragg grating structure as a complement to the planar Bragg mirrors of a conventional, high Q semiconductor cavity. Such circular Bragg gratings do not form the initial resonant cavity, but rather, aid in controlling relatively low Q, transverse modes of an existing Fabry-Perot structure. In such cases, the resultant control of transverse propagation may allow lowered thresholds, or enhanced stability.
Earlier, large-scale, laser designs of a circular geometry operated on very different principles than the microlasers, utilizing primarily gas laser mediums and metallic reflectors. In these earlier designs, optical power could be coupled for useful work at the center of the cavity, such as for isotope separation, or by using a conical reflector. Since, in these latter cases, laser modes that concentrated energy at the cavity""s center were needed, some means for blocking the whispering-type modes was generally required. Such mode suppression was usually accomplished through radial stops; however, these stops only provided the most rudimentary mode control, in addition to hampering the efficient operation of the laser. Because of such issues, disk and spherical lasers have not supplanted standard linear lasers for any applications requiring substantial optical power or a high degree of mode selection.
A novel laser apparatus has been developed for use in such applications as lasers and light amplifiers in general. The laser developed comprises a cavity mirror structure that provides a single surface of revolution. The cavity volume is defined by this surface of revolution, and contains the gain medium. Unlike prior art disk and/or spherical lasers possessing circular cavities, the present invention does not rely on total internal reflection (TIR) or metallic reflectors to provide a high cavity Q-factor (and a broad range of high-order modes). The laser design of the present invention avoids use of these cavity confinement methods. In the optical resonator of the present invention, interference-based multilayer dielectric (MLD) reflectors are constructed that can possess unusually narrow reflection peaks, corresponding to a degree of finesse (finesse designating interference-based resolving power) usually associated with MLD transmission filters of the Fabry-Perot type. The high-finesse MLD reflectors of the present invention conform to the surface of revolution of the cavity mirror structure, allowing a high degree of angle-dependence for selective containment of cavity modes. These filters are disposed in such a way as to allow preferred-low order modes (lower order modes being represented in the present disclosure as those corresponding to near normal incidence radiation) and suppression of parasitic modes while allowing a high cavity Q factor for the modes selected.
For a multi-layer dielectric (MLD) coating consisting of alternating layers, where all layers have an optical thickness equal to a quarter-wave of light at the wavelength of interest, the reflectance may be described according to:                     R        =                              [                                          1                -                                                                            (                                                                        n                          H                                                /                                                  n                          L                                                                    )                                                              2                      ⁢                      p                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  n                        H                        2                                            /                                              n                        L                                                              )                                                                              1                +                                                                            (                                                                        n                          H                                                /                                                  n                          L                                                                    )                                                              2                      ⁢                      p                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  n                        H                        2                                            /                                              n                        L                                                              )                                                                        ]                    2                                    (        1        )            
wherein the index of refraction for the substrate is ns, the two layer indices are nH (high index) and nL (low index), and the number of pairs of alternating layers is p. As is evidenced by equation (1), a higher reflectance may be achieved through the implementation of a greater difference in refractive index xcex94n=|n2xe2x88x92n1|. High reflectance is thus normally achieved by maintaining xcex94n at a relatively high value. However, as equation (1) suggests, high reflectance may also be achieved by depositing many layer pairs possessing a relatively low difference in their refractive indices. As the index difference decreases, many more pairs of alternating layers must be deposited to maintain reasonable reflectance. At the same time, this latter approach will result in a decrease in the bandwidth of light reflected by the resultant coating. The present invention utilizes MLD coatings which obtain high reflectance from an unusually low xcex94n; this is accomplished by maintaining a high degree of control over the properties of each layer through an unusually high number of iterations, p, of the layer pair. With well-controlled film characteristics, the reflectance of the resulting MLD coating is found to have a quite narrow bandwidth, typically in the order of nanometers.
A characteristic of the MLD coatings utilized in the present invention is the angle-dependence of the reflection peak. As the MLD coating is irradiated at increasingly oblique angles of incidence, the spectrally narrow reflection peak will be shifted toward increasingly shorter wavelengths. While the degree of this latter peak shift will depend on such issues as phase dispersion and the change in optical admittance with increasingly oblique incidence, the fractional shift in the peak transmittance will change generally with the phase thickness shift. As such, the fractional shift in peak transmittance will be slightly less than cos xcex8,where xcex8 is the angle from normal incidence. As the angle of incidence, xcex8, increases, the magnitude of the reflectance peak will generally decrease, as well.
The aforementioned characteristics of these high-finesse MLD coatings are utilized in the preferred embodiments of the present invention. In accordance with the illustrated preferred embodiments, a novel laser cavity structure is disclosed herein that effectively utilizes the sensitivity of the aforementioned coatings to angle-of-incidence when these same coatings are irradiated with quasi-monochromatic light. This is normally accomplished through the use of a cavity mirror that conforms to a single surface of revolution. High confinement is achieved through novel use of the highly angle-dependent MLD reflectors. Thus, instead of utilizing TIR or metal films, which both provide wide acceptance angles to high order cavity modes, the present invention utilizes external reflection and narrow acceptance angles to increase the stability of selected, lower order, cavity modes.
Because the present invention does not rely on TIR or metallic films to provide high confinement for various laser modes, it is designed with a fundamentally different set of requirements for the refractive indices of its individual components. In contrast to the disk and spherical lasers of the prior art, the gain mediumxe2x80x94or, equivalently, the volume in which it residesxe2x80x94in lasers of the present invention should possess an effective refractive index, nG, lower than that of the immediately surrounding medium. As such, the high index layers of the MLD of the present invention must have a refractive index, nH, greater than that of the gain volume.
In one preferred embodiment, the present invention provides a laser cavity structure that does not require a partially reflective mirror or external optics to efficiently couple laser light to a work piece or various process media. Instead, the laser cavity structure disclosed herein allows photo-absorbing media to be introduced through the center of the cavity, so that energy not absorbed by the photo-absorbing media may contribute back to the energy stored inside the cavity. According to this aspect, the irradiation of photo-absorbing media may also be rendered highly uniform, and is well suited for media of substantially circular symmetry.
In another embodiment, the invention provides a unique configuration for coupling laser radiation from the edge of the spherical and disk lasers described, as the mode selection provided allows efficient coupling of a low-divergence beam from the cavity edge. Other objects of the present invention follow.
One objective of the present invention is to provide a laser cavity structure that allows high thermal stability.
Another objective of the present invention is to provide a disk or spherical laser cavity structure that discourages the establishment of whispering modes
Another object of the present invention is to provide a laser cavity structure which allows mode selection through the use of all-dielectric reflectors of unusually high finesse.
Yet another object of the present invention is to increase the stability of conventional laser cavity structures through the suppression of walk-off modes.
Another object of the present invention is to provide a laser cavity structure that allows a low threshold to lasing.
Another object of the present invention is to provide a means for irradiating a photo-absorbing medium from a continuous 360-degree periphery.
Another object of the present invention is to provide a laser cavity structure that allows efficient and reliable mechanical design.